The invention generally relates to radiographic imaging and, more particularly, relates to a method and apparatus for reading a computed radiography phosphor plate or sheet that has been exposed by X rays by supplying pumping light thereto.
It is well known that, by using X-ray systems, features can be visualized within the human body or within industrial products, or the like. Current X-ray systems often use X-ray film which must be developed.
In the alternative, computed tomography installations are available but are very expensive and require large amounts of computer power.
In addition systems exist which use a technique called computed radiography. A patient is exposed with X rays and a latent X-ray image is formed on a phosphor-containing computed radiography plate or sheet that is similar to a sheet of film. The phosphor-containing sheet typically may include a rare earth, such as europium, in combination with barium and fluorine. Other sheet formulations also are available. The sheet is sensitive to X rays and can store a latent X-ray image thereon. Because the sheet is also sensitive to light it is kept in the dark. A sheet containing a latent X-ray image is imaged in a scanner by exposing the sheet and its latent image to a raster-scanned laser beam. Areas of the sheet which have preferentially received X-ray energy phosphoresce, making the latent X-ray image visible.
While the scanner is convenient and allows reuse of the computed radiography sheets multiple numbers of times, it does suffer from certain drawbacks. It is difficult to obtain a high-spatial resolution image because the pumping laser beam, although only covering a small spot-size at a time, tends to leave illumination energy behind, which causes bloom; thereby smearing the image and reducing its resolution. This is because the image is built up in the way that an image would be in a flying spot device wherein only a single optical detector is used. The single optical detector can capture radiation from almost any position on the sheet. The optical detector, however, is unable to determine whether the photons it is receiving are coming from unwanted bloom or coming from active phosphorescence caused by excitation by the laser beam.
In addition the existing systems either operate in the laser visible region at about 630 to 650 nanometers or, in the near infrared region, at about 940 nanometers.
A single laser cannot be used for both wavelengths. Because there are differing types of latent imaging materials used for computed radiography, not all phosphoresce either with red pumping light or with infrared pumping light. A scanner which uses a pumping laser in either the red or infrared region cannot accept plates or sheets having latent images which must be optically pumped in the other region.
The prior raster-scanned laser systems introduce spatial non-linearities in the image for which there must be compensation. The non-linearities are due to the difference in the effective beam scan rate when the beam is substantially perpendicular to the latent image containing sheet at the center portion of the sheet and when it is sweeping at an angle to the sheet near the sheet edges. As a result, since the image is constructed based upon on pumping beam timing and orientation, elaborate methods would have to be used in order to effectively relinearize the beam scan to provide an undistorted image.
What is needed, then, is a system and apparatus which can quickly and conveniently provide highly-accurate and high resolution computed radiography visible images without the need for expensive equipment.
The present invention is embodied in an apparatus and method for radiographic imaging wherein a substrate comprising a computed radiography plate or sheet is exposed to X rays to form a latent image thereon. The apparatus comprises an optical pump source which is a plurality of light emitting diodes (LEDs). The LEDs emit light at two visible wavelengths and one infrared wavelength. The pumping light from the LEDs is supplied to a plurality of transmit optical fibers which deliver the pumping light to the computed radiography sheet being scanned.
Alternatively, a laser carried on a rotating platform can sequentially illuminate ends of the transmit fibers to supply coherent pumping light thereto.
The transmit optical fibers have their delivery ends aligned in a linear array adjacent the position at which they deliver pumping light to the computed radiography sheet. A motor causes the sheet to be moved under the transmit linear fiber array as the sheet is exposed to the pumping light from the transmit fiber ends. In addition, when the LEDs are used as the illumination source the transmit fibers are multiplexed in groups of sixty four, to provide relatively wide spacing between transmit fiber ends that are simultaneously pumping light to the sheet. This avoids bloom from one excitation or pumping fiber to the next at any one time and improves the optical resolution provided by the pumping light.
A second plurality of optical fibers comprises receive optical fibers, each having a diameter of about 500 microns collects the emitted light and supplies it to photodiodes or other optical transducers, such as a photomultiplier tube, which generate an image signal representative of light intensity. That signal is supplied to a processor which generates an image signal. The image signal may then be used to generate a visible image representative of the latent x-ray image on the radiographic substrate.
In a further embodiment of the present invention the apparatus will include a unitary light pipe comprised of a single piece of substantially transparent plastic although glass or other transparent material can be substituted. The light pipe can collect all light available along a scan line at the computed radiography plate and carry it to a photodetector, usually a photomultiplier, for conversion to an electrical signal. With this type of construction most of the intermediate optics found in prior art computed radiography plate scanning systems is avoided. Many problems associated with optical misalignment, dust, vibration, leading to temporary misalignment, and lack of scan linearity is reduced if not eliminated.
In addition, the only moving parts, effectively speaking in the optical train are the plate feeding mechanism and the laser. No other of the optical components are separately movable which might lead to misalignment problems.
A further advantage of the present invention is that the system allows the use of standard power and networking interfaces to allow easy transfer of information from the system to a personal computer such as a laptop computer for generation of an image. The apparatus also can be used as part of a larger radiography system should it be so desired.
It is a principal aspect of the present invention to provide a high resolution radiographic imaging apparatus.
Other aspects and advantages of the present invention will become obvious to one of ordinary skill in the art upon a perusal of the following specification and claims in light of the accompanying drawings.